Ability to provide SSH key

Related to bunch of users already reporting [issue cloning private Github Repo](https://github.com/aws-amplify/amplify-console/issues/157#issuecomment-834362708)

Would it be possible to implement a feature where user of AWS Amplify would be able to provide SSH private key (E.g in env variable or some Amplify config)

This way Amplify user can add public part of the key to his Github user / bot user

This private key could then be injected to the docker build container (e.g. [like this](https://nrogap.medium.com/docker-and-how-to-add-an-ssh-key-to-a-container-d9b72f1a6a46) so that when cloning repo from github Amplify would use the private key the Amplify user want to use

This way you will ensure there is s no friction cloning Github repositories.

something similar was attempted [in this article](https://sapanbodiwala.com/blog/deploying-to-aws-amplify) but problem is the cloning fails before the `preBuild` step

Trace the GitHub repository cloning flow and identify what runs before the preBuild step, including how the Docker build container is configured. Define how users would provide an SSH private key securely and verify that cloning private repositories succeeds before preBuild runs.

Written by the indexing model from the issue text.